8

®

OPA686

APPLICATIONS INFORMATION

WIDEBAND, NON-INVERTING OPERATION

The OPA686 provides a unique combination of features—

low input voltage noise along with a very low distortion

output stage—to give one of the highest dynamic range op

amps available. Its very high Gain Bandwidth Product (GBP)

can be used either to deliver high signal bandwidths at high

gains, or to deliver very low distortion signals at moderate

frequencies and lower gains. To achieve the full perfor-

mance of the OPA686, careful attention to PC board layout

and component selection is required as discussed in the

remaining sections of this data sheet.

Figure 1 shows the non-inverting gain of +10 circuit used as

the basis of the Electrical Specifications and most of the

Typical Performance Curves. Most of the curves were char-

acterized using signal sources with 50

Ω driving impedance,

and with measurement equipment presenting a 50

Ω load

impedance. In Figure 1, the 50

Ω shunt resistor at the V

I

terminal matches the source impedance of the test generator,

while the 50

Ω series resistor at the V

matching resistor for the measurement equipment load.

Generally, data sheet voltage swing specifications are at the

specifications are at the matched 50

Ω load. The total 100Ω

load at the output, combined with the 503

Ω total feedback

network load, presents the OPA686 with an effective output

load of 83

Ω for the circuit of Figure 1.

Voltage feedback op amps, unlike current feedback designs,

can use a wide range of resistor values to set their gain. The

circuit of Figure 1, and the specifications at other gains, use

adjusted to get the desired gain. Using this guideline will

guarantee that the noise added at the output due to Johnson

noise of the resistors will not significantly increase the total

noise over that due to the 1.3nV/

√Hz input voltage noise for

the op amp itself.

WIDEBAND, INVERTING GAIN OPERATION

Operating the OPA686 as an inverting amplifier has several

benefits and is particularly appropriate when a matched

input impedance is required. Figure 2 shows the inverting

gain circuit used as the basis of the inverting mode Typical

Performance Curves.

FIGURE 1. Non-Inverting, G = +10 Specification and Test

Circuit.

FIGURE 2. Inverting, G = –20 Characterization Circuit.

Driving this circuit from a 50

Ω source, and constraining the

termination resistor and the gain setting resistor for the

Figure 2 is double that for Figure 1, the noise gains are in

fact equal when the 50

Ω source resistor is included. This

has the interesting effect of doubling the equivalent GBP of

the amplifier. This can be seen in comparing the G = +10

and G = –20 small-signal frequency response curves. Both

show approximately 250MHz bandwidth, but the inverting

configuration of Figure 2 gives 6dB higher signal gain. If

the signal source is actually the low impedance output of

should be adjusted to achieve the desired gain. For stable

operation of the OPA686, it is critical that this driving

amplifier show a very low output impedance at frequencies

beyond the expected closed-loop bandwidth for the OPA686.

WIDEBAND, HIGH SENSITIVITY, TRANSIMPEDANCE

DESIGN

The high Gain Bandwidth Product (GBP) and the low input

voltage and current noise for the OPA686 make it an ideal

wideband transimpedance amplifier for low to moderate

transimpedance gains. Very high transimpedance gains

(> 100k

Ω) will benefit from the low input noise current of

a FET-input op amp such as the OPA655. Unity gain

stability in the op amp is not required for application as a

OPA686

+5V

–5V

–V

S

+V

S

50

Ω

V

O

V

I

50

Ω

+

0.1µF

+

6.8µF

6.8µF

R

G

50

Ω

R

F

453

Ω

50

Ω Source

50

Ω Load

0.1µF

OPA686

+5V

–5V

+V

S

–V

S

91

Ω

50

Ω

V

O

V

I

+

6.8µF

0.1µF

+

6.8µF

0.1µF

0.1µF

R

F

1k

Ω

R

G

50

Ω

50

Ω Source

50

Ω Load